Microelectromechanical systems (MEMS) variable capacitive devices are components implemented in a number of systems including, for example, reconfigurable radio-frequency (RF) systems. MEMS variable capacitive devices are tunable capacitors that enable either binary states of capacitance (e.g., C1 and C2) or continuously variable states of capacitance over a range of values. In a tunable parallel plate capacitor, change in capacitance is effected by varying the distance between the metal layers forming the capacitor plates via an actuation method, such as piezoelectric and/or electrostatic actuation. Upon actuation, the distance between the capacitor plates is tuned via one or more springs, to which one of the plates is attached. Of increasing interest is the implementation of piezoelectric capacitors in low power consumption, miniaturized systems due to their low actuation voltage requirements and low consumption power.
FIG. 1 shows a side view of a prior art MEMS capacitive device 20 formed using a single substrate surface micromachining process. Capacitive device 20 includes a substrate 22 having a surface 24. A fixed capacitor plate 26 is formed on surface 24. A movable element 32 includes first and second ends 34 and 36, respectively, also coupled to surface 24 of substrate 22. An intermediate section 38 of movable element 32 spans between first and second ends 34 and 36.
A movable metal capacitor plate 40 is formed on an inner surface 42 of intermediate section 38 overlying fixed capacitor plate 26. A pair of piezoelectric actuators 44 and 46 are also formed on inner surface 42 of intermediate section 38. Actuation of actuators 44 and 46 causes intermediate section 38 to flex toward surface 24 of substrate 22, thereby altering a capacitance 48 between a contact surface 49 of fixed capacitor plate 26 and movable capacitor plate 40.
MEMS capacitive device 20 is also illustrated to include a pair of elements 28 and 30 formed on surface 24. Elements 28 and 30 are arranged on opposing sides of fixed capacitor plate 26, with element 28 facing actuator 44 and element 30 facing actuator 46. Elements 28 and 30 shown in FIG. 1 reflect optional electrodes which could be used with actuators 44 and 46 to cause intermediate section 38 to flex toward surface 24 of substrate 22 by electrostatic actuation. Actuators 44 and 46 could then be either piezoelectric actuators, electrostatic actuator electrodes, or a combination of both.
One difficulty in designing and fabricating a MEMS parallel plate capacitor, such as capacitive device 20, is to achieve acceptable process tolerance while concurrently achieving successful device reproducibility in mass production. This difficulty is exacerbated when using piezoelectric actuation in the MEMS capacitor. For example, actuators 44 and 46 may be piezoelectric actuators, in order to achieve the advantages of low actuation voltage requirements and low consumption power. However, fabrication processes for piezoelectric material requires high temperature annealing (e.g., six hundred to seven hundred degrees Celsius). The high temperature processing can result in stress and thermal mismatch effects that adversely affect capacitor yield and capacitor accuracy in a single substrate device, such as MEMS capacitive device 20.
FIG. 2 shows a side view of a prior art MEMS capacitive device 50 formed using surface micromachining and substrate bonding processes. Capacitive device 50 includes a substrate 52 having a surface 54. A fixed capacitor plate 56 is formed on surface 54. Capacitive device 50 further includes another substrate 62 bonded to substrate 52. A movable element 66 includes first and second ends 68 and 70, respectively, coupled to a surface 64 of substrate 62. An intermediate section 72 of movable element 66 spans between first and second ends 68 and 70.
A movable metal capacitor plate 74 is formed on an outer surface 76 of intermediate section 72 overlying fixed capacitor plate 56. A pair of piezoelectric actuators 78 and 80 are also formed on outer surface 76 of intermediate section 72. Actuation of actuators 78 and 80 causes intermediate section 72 to flex toward surface 54 of substrate 52, thereby altering a capacitance 82 between fixed capacitor plate 56 and a contact surface 84 of movable capacitor plate 74.
MEMS capacitive device 50 is also illustrated to include a pair of elements 58 and 60 formed on surface 54. Elements 58 and 60 are arranged on opposing sides of fixed capacitor plate 56, with element 58 facing actuator 78 and element 60 facing actuator 80. Elements 58 and 60 shown in FIG. 2 reflect optional electrodes which could be used with actuators 78 and 80 to cause intermediate section 72 to flex toward surface 54 of substrate 52 by electrostatic actuation. Actuators 78 and 80 could then be either piezoelectric actuators, electrostatic actuator electrodes, or a combination of both.
In this capacitive device 50, the issues of stress and thermal mismatch are mitigated by forming piezoelectric actuators 78 and 80 separate from the underlying fixed capacitor plate 56. However, a surface micromachining process is a build-up technique based on the deposition and etching of different structural and sacrificial layers on top of the substrate. Consequently, contact surface 49 (FIG. 1) of capacitor plate 40 (FIG. 1) is exposed following removable of a sacrificial layer (not shown). In contrast, movable metal capacitor plate 74 is formed as a top layer, and contact surface 84 is a “top” surface.
A sacrificial layer can be deposited to have a very smooth surface. As such, an overlying structural layer, such as contact surface 49 of capacitor plate 40 can be commensurately smooth. However, contact surface 84 of capacitor plate 74 is rough relative to that of contact surface 49 due to the grain size of the metal of capacitor plate 74. Unfortunately, the roughness of contact surface 84 leads to poor reproducibility.
Accordingly, there is a need for an improved MEMS capacitive device and fabrication methodology for overcoming the problems in the art as discussed above.